Evidence of splinting in low back pain? A systematic review of perturbation studies.

PURPOSE: The purpose of this systematic review was to assess whether LBP patients demonstrate signs of splinting by evaluating the reactions to unexpected mechanical perturbations in terms of (1) trunk muscle activity, (2) kinetic and (3) kinematic trunk responses and (4) estimated mechanical properties of the trunk. METHODS: The literature was systematically reviewed to identify studies that compared responses to mechanical trunk perturbations between LBP patients and healthy controls in terms of muscle activation, kinematics, kinetics, and/or mechanical properties. If more than four studies reported an outcome, the results of these studies were pooled. RESULTS: Nineteen studies were included, of which sixteen reported muscle activation, five kinematic responses, two kinetic responses, and two estimated mechanical trunk properties. We found evidence of a longer response time of muscle activation, which would be in line with splinting behaviour in LBP. No signs of splinting behaviour were found in any of the other outcome measures. CONCLUSIONS: We conclude that there is currently no convincing evidence for the presence of splinting behaviour in LBP patients, because we found no indications for splinting in terms of kinetic and kinematic responses to perturbation and derived mechanical properties of the trunk. Consistent evidence on delayed onsets of muscle activation in response to perturbations was found, but this may have other causes than splinting behaviour.

Systemic Inflammation, Sleep, and Psychological Factors Determine Recovery Trajectories for People With Neck Pain: An Exploratory Study.

We conducted an explorative prospective cohort study with 6 months follow-up to 1) identify different pain and disability trajectories following an episode of acute neck pain, and 2) assess whether neuroimmune/endocrine, psychological, behavioral, nociceptive processing, clinical outcome, demographic and management-related factors differ between these trajectories. Fifty people with acute neck pain (ie, within 2 weeks of onset) were included. At baseline, and at 2, 4, 6, 12, and 26 weeks follow-up, various neuroimmune/endocrine (eg, inflammatory cytokines and endocrine factors), psychological (eg, stress symptoms), behavioral (eg, sleep disturbances), nociceptive processing (eg, condition pain modulation), clinical outcome (eg, trauma), demographic factors (eg, age), and management-related factors (eg, treatment received) were assessed. Latent class models were performed to identify outcome trajectories for neck pain and disability. Linear mixed models or the Pearson chi-square test were used to evaluate differences in these factors between the trajectories at baseline and at each follow-up assessment and over the entire 6 months period. For pain, 3 trajectories were identified. The majority of patients were assigned to the "Moderate pain - Favourable recovery" trajectory (n = 25; 50%) with smaller proportions assigned to the "Severe pain - Favourable recovery" (n = 16; 32%) and the "Severe pain - Unfavourable recovery" (n = 9; 18%) trajectories. For disability, 2 trajectories were identified: "Mild disability - Favourable recovery" (n = 43; 82%) and "Severe disability - Unfavourable recovery" (n = 7; 18%). Ongoing systemic inflammation (increased high-sensitive C-reactive protein), sleep disturbances, and elevated psychological factors (such as depression, stress and anxiety symptoms) were mainly present in the unfavorable outcome trajectories compared to the favorable outcome trajectories. PERSPECTIVE: Using exploratory analyses, different recovery trajectories for acute neck pain were identified based on disability and pain intensity. These trajectories were influenced by systemic inflammation, sleep disturbances, and psychological factors.

The effects of knee arthroplasty on walking speed: a meta-analysis.

BACKGROUND: Patients with knee osteoarthritis patients have problems with walking, and tend to walk slower. An important aim of knee arthroplasty is functional recovery, which should include a post-operative increase in walking speed. Still, there are several problems with measuring walking speed in groups of knee osteoarthritis patients. Nevertheless, test-retest reliability of walking speed measurements is high, and when the same investigators monitor the same subjects, it should be possible to assess the walking speed effects of knee arthroplasty. The present study reports a meta-analysis of these effects. METHODS: A total of 16 independent pre-post arthroplasty comparisons of walking speed were identified through MEDLINE, Web of Science, and PEDro, in 12 papers, involving 419 patients. RESULTS: For 0.5-5 months post-operatively, heterogeneity was too large to obtain a valid estimate of the overall effect-size. For 6-12 and 13-60 months post-operatively, heterogeneity was absent, low, or moderate (depending on estimated pre-post correlations). During these periods, subjects walked on average 0.8 standard-deviations faster than pre-operatively, which is a large effect. Meta-regression analysis revealed significant effects of time and time squared, suggesting initial improvement followed by decline. CONCLUSION: This meta-analysis revealed a large effect of arthroplasty on walking speed 6-60 months post-operatively. For the first 0.5-5 months, heterogeneity of effect-sizes precluded a valid estimate of short-term effects. Hence, patients may expect a considerable improvement of their walking speed, which, however, may take several months to occur. Meta-regression analysis suggested a small decline from 13 months post-operatively onwards.

Strong relationship of muscle force and fall efficacy, but not of gait kinematics, with number of falls in the year after Total Hip Arthroplasty for osteoarthritis: An exploratory study.

BACKGROUND: In people with moderate hip osteoarthritis, gait kinematics was reported to be correlated with number of falls in the preceding year. After Total Hip Arthroplasty, subjects generally improve but still fall. The present study explores recovery and correlations with number of falls in the year after Total Hip Arthroplasty. METHODS: We assessed 12 patients one year after Total Hip Arthroplasty, 12 patients with moderate hip osteoarthritis with at least one fall in the preceding year, and 12 healthy peers. Maximum hip abduction strength, Fall Efficacy Scale - International, Harris Hip Score, pain, and number of falls in the preceding year were assessed. Participants walked on a treadmill with increasing speeds, and gait kinematics were registered optoelectronically. We assessed group differences, and correlations of all variables with number of falls. FINDINGS: After arthroplasty, subjects tended to score better on variables measured, often non-significantly, compared to subjects with moderate osteoarthritis, but worse than healthy peers. Maximum hip abduction strength together with fall efficacy had a strong regression on the number of falls in the preceding year (R2 = 92%). Gait kinematics did not correlate with number of falls, and also fall efficacy was not related to gait kinematics. INTERPRETATION: One year after hip arthroplasty, muscle strength sufficiently recovered for normal walking, but not to avoid falling in risky situations. Rehabilitation should focus on muscle strength. The lack of correlation between the Fall Efficacy International and gait kinematics, suggests that it reflected the experience of having fallen rather than fear.

Gait adaptations in low back pain patients with lumbar disc herniation: trunk coordination and arm swing.

Patients with chronic non-specific low back pain (LBP) walk with more synchronous (in-phase) horizontal pelvis and thorax rotations than controls. Low thorax-pelvis relative phase in these patients appears to result from in-phase motion of the thorax with the legs, which was hypothesized to affect arm swing. In the present study, gait kinematics were compared between LBP patients with lumbar disc herniation and healthy controls during treadmill walking at different speeds and with different step lengths. Movements of legs, arms, and trunk were recorded. The patients walked with larger pelvis rotations than healthy controls, and with lower relative phase between pelvis and thorax horizontal rotations, specifically when taking large steps. They did so by rotating the thorax more in-phase with the pendular movements of the legs, thereby limiting the amplitudes of spine rotation. In the patients, arm swing was out-of phase with the leg, as in controls. Consequently, the phase relationship between thorax rotations and arm swing was altered in the patients.

Is the psoas a hip flexor in the active straight leg raise?

Psoas function is a topic of considerable relevance in sports and clinical science. However, the literature on psoas function is not sufficiently consistent. Questions are, amongst others, if during hip flexion the psoas always has the same function as the iliacus, and if the psoas affects the hip more than the lumbar spine. In the present study, 17 healthy women, 20-40 years, performed the active straight leg raise (ASLR), with the right or the left leg ("Side"), and without or with weight added above the ankle ("Condition"). Electromyographic (EMG) activity of psoas and iliacus were recorded with fine-wire electrodes, and of rectus femoris and adductor longus with surface electrodes, all on the right side. Movements of the leg were recorded with active markers and a camera system. During ASLR, the iliacus, rectus femoris, adductor longus and psoas were active ipsilaterally, but psoas was also active contralaterally. All muscles started to contract before movement onset, the iliacus, rectus femoris, and adductor longus largely at the same time, before the psoas. There was no significant difference between the amplitude or time of onset of ipsilateral and contralateral psoas EMG activity, nor was there a significant interaction between Side and Condition for the psoas. Although ipsilateral psoas activity is consistent with the psoas being a hip flexor, contralateral activity is not. The most simplest explanation of the pattern found is that the psoas is bilaterally recruited to stabilize the lumbar spine, probably in the frontal plane.

Effects of joint and nerve mobilisation on neuroimmune responses in animals and humans with neuromusculoskeletal conditions: a systematic review and meta-analysis.

Several animal and human studies revealed that joint and nerve mobilisations positively influence neuroimmune responses in neuromusculoskeletal conditions. However, no systematic review and meta-analysis has been performed. Therefore, this study aimed to synthesize the effects of joint and nerve mobilisation compared with sham or no intervention on neuroimmune responses in animals and humans with neuromusculoskeletal conditions. Four electronic databases were searched for controlled trials. Two reviewers independently selected studies, extracted data, assessed the risk of bias, and graded the certainty of the evidence. Where possible, meta-analyses using random effects models were used to pool the results. Preliminary evidence from 13 animal studies report neuroimmune responses after joint and nerve mobilisations. In neuropathic pain models, meta-analysis revealed decreased spinal cord levels of glial fibrillary acidic protein, dorsal root ganglion levels of interleukin-1ß, number of dorsal root ganglion nonneuronal cells, and increased spinal cord interleukin-10 levels. The 5 included human studies showed mixed effects of spinal manipulation on salivary/serum cortisol levels in people with spinal pain, and no significant effects on serum ß-endorphin or interleukin-1ß levels in people with spinal pain. There is evidence that joint and nerve mobilisations positively influence various neuroimmune responses. However, as most findings are based on single studies, the certainty of the evidence is low to very low. Further studies are needed.

The short- and long-term temporal relation between falls and concern about falling in older adults without a recent history of falling.

BACKGROUND AND AIM: The reciprocal relation between falling and concern about falling is complex and not well understood. We aimed to determine whether concern about falling increases after a fall and whether concern about falling increases the odds of future falls in community-dwelling older adults without a recent fall history. METHODS: We selected 118 community-dwelling older adults (mean age: 71.4 (SD: 5.3) years) without a self-reported history of falling, one year prior to baseline assessment, from the one-year VIBE cohort for analyses. On a monthly basis, we recorded concern about falling (using the Falls Efficacy Scale-International, FES-I), as well as the occurrence of falls (through questionnaires and telephone calls). We determined 1) whether falling predicts an increase in concern about falling and 2) whether a high concern about falling is predictive of falling. Standard linear (fixed-effects) regression and mixed effects regression analyses were performed over long-term, i.e. one year, and short-term, i.e. one-month, intervals, respectively and were adjusted for gender, age and physical activity (quantified as the average total walking duration per day). Analyses were performed separately for all reported falls and for injurious falls only. RESULTS: High concern about falling at baseline did not predict falls over the course of one year, nor over the course of one month. Furthermore, falls in between baseline assessment and one year thereafter did not predict increased concern about falling from baseline to one year later, independent of whether all falls or only injurious falls were considered. However, falls, either all or injurious only, happening somewhere over the course of a one-month interval, significantly predicted small increases in concern about falling (1.49 FES-I points, 95% CI [0.74, 2.25], p<0.001 for all falls; 2.60 FES-I points, 95% CI [1.55, 3.64], p<0.001 for injurious falls) from the start to the end of that one-month interval. CONCLUSION: Older adults without a recent history of falling seem to be resilient against developing concern about falling after having fallen, resulting in a short-term temporary effect of falling on concern about falling. Furthermore, we found no evidence that a high concern about falling predicts future falls over a one-month or a one-year follow-up period, suggesting that concern is not a primary cause for falls in older adults without a recent history of falling.

Coordination of Axial Trunk Rotations During Gait in Low Back Pain. A Narrative Review.

Chronic low back pain patients have been observed to show a reduced shift of thorax-pelvis relative phase towards out-of-phase movement with increasing speed compared to healthy controls. Here, we review the literature on this phase shift in patients with low back pain and we analyze the results presented in literature in view of the theoretical motivations to assess this phenomenon. Initially, based on the dynamical systems approach to movement coordination, the shift in thorax-pelvis relative phase with speed was studied as a self-organizing transition. However, the phase shift is gradual, which does not match a self-organizing transition. Subsequent emphasis in the literature therefore shifted to a motivation based on biomechanics. The change in relative phase with low back pain was specifically linked to expected changes in trunk stiffness due to 'guarded behavior'. We found that thorax-pelvis relative phase is affected by several interacting factors, including active drive of thorax rotation through trunk muscle activity, stride frequency and the magnitude of pelvis rotations. Large pelvis rotations and high stride frequency observed in low back pain patients may contribute to the difference between patients and controls. This makes thorax-pelvis relative phase a poor proxy of trunk stiffness. In conclusion, thorax-pelvis relative phase cannot be considered as a collective variable reflecting the orderly behaviour of a complex underlying system, nor is it a marker of specific changes in trunk biomechanics. The fact that it is affected by multiple factors may explain the considerable between-subject variance of this measure in low back pain patients and healthy controls alike.

The effects of arm swing on human gait stability.

Arm swing during human gait has been shown to reduce both angular momentum about the vertical and energy expenditure, and has been hypothesized to enhance gait stability. To examine this hypothesis, we studied the effect of arm swing on the local and global stability of steady-state gait, as well as the ability to perform adequate recovery actions following a perturbation. Trunk kinematics of 11 male subjects was measured in treadmill walking with normal and with restricted arm swing. In half of the trials, gait was perturbed by a position-controlled forward pull to the trunk. We constructed state spaces using data recorded from the unperturbed steady-state walking trials, and quantified local gait stability by calculating maximum Lyapunov exponents. In addition, we analyzed perturbation forces, the distance from the unperturbed gait pattern, and the return toward the normal gait pattern following an external perturbation. Walking without arm swing led to a non-significantly lower Lyapunov exponent (P=0.06), significantly higher perturbation forces (P<0.05), and significantly slower movements away from the attractor (P<0.01). These results suggest that gait without arm swing is characterized by similar local stability to gait with arm swing and a higher perturbation resistance. However, return towards the normal gait pattern was significantly slower (P<0.05) when walking with restricted arms, suggesting that the arms play an important role in the recovery from a perturbation. Collectively, the results suggest that arm swing as such does not enhance gait stability, but rather that recovery movements of the arms contribute to the overall stability of human gait.

Axial pelvis range of motion affects thorax-pelvis timing during gait.

During gait, patients with pelvic girdle pain and low back pain demonstrate an altered phase relationship between axial thorax and pelvis rotations (thorax-pelvis relative phase). This could be the result of an increase in axial pelvis range of motion (ROM) which has been observed in these patients as well. To establish this relationship, we investigated if altered axial pelvis ROM during gait affects thorax-pelvis relative phase in 12 healthy subjects. These subjects walked on a treadmill and received real-time feedback on axial pelvis rotations. Subjects were asked to (1) walk normal, and walk with (2) decreased and (3) increased pelvis ROM. Gait speed and stride frequency were matched between trials. Subjects were able to increase pelvis ROM to a large extent, but the reduction in pelvis ROM was relatively small. Walking with large pelvis ROM resulted in a change in thorax-pelvis relative phase similar to that in pelvic girdle pain and low back pain. A forward dynamic model was used to predict the effect of manipulation of pelvis ROM on timing of thorax rotations independent of apparent axial trunk stiffness and arm swing amplitude (which can both affect thorax-pelvis relative phase). The model predicted a similar, even larger, effect of large axial pelvis ROM on thorax-pelvis relative phase, as observed experimentally. We conclude that walking with actively increased ROM of axial pelvis rotations in healthy subjects is associated with a shift in thorax-pelvis relative phase, similar to observations in patients with pelvic girdle pain and low back pain.

Axial Thorax-Pelvis Coordination During Gait is not Predictive of Apparent Trunk Stiffness.

The coordination of axial thorax and pelvis rotations during gait has been shown to be affected by several pathologies. This has been interpreted as an indication of increased apparent axial trunk stiffness, but arm swing may also affect these rotations. The objectives of this study were to assess the effect of trunk stiffness and arm swing on the relative timing ('coordination') between thorax and pelvis rotations, and to assess if apparent trunk stiffness can be inferred from thorax-pelvis kinematics. A forward dynamic model was constructed to estimate apparent trunk stiffness from observed thorax and pelvis rotations and arm swing moment around the longitudinal axis of the trunk of 30 subjects. The effect of independent manipulations of trunk stiffness and arm swing moment on thorax-pelvis coordination and gain of axial thorax-pelvis rotations were assessed using the same forward dynamic model. A linear regression model was constructed to evaluate whether forward dynamic model-based estimates of axial trunk stiffness could be inferred directly from thorax-pelvis rotations. The forward dynamic model revealed that axial trunk stiffness and arm swing moment have opposite effects on axial thorax-pelvis coordination. Apparent axial trunk stiffness could not be predicted from observed thorax-pelvis rotations.

Coordination of leg swing, thorax rotations, and pelvis rotations during gait: the organisation of total body angular momentum.

In walking faster than 3 km/h, transverse pelvic rotation lengthens the step ("pelvic step"). It is often assumed that the thorax then starts to counter rotate to limit total body angular momentum around the vertical. But the relative timing of pelvis and thorax rotation during gait is insufficiently understood. The present study aimed at analysing how transverse pelvis and thorax rotations relate to the movements of the upper leg, and how these patterns contribute to total body angular momentum. Nine healthy male volunteers walked on a treadmill at nine different velocities, ranging from 2.0 km/h to 5.2 km/h. Full body kinematics were recorded. Femur-pelvis, pelvis-thorax, and femur-thorax relative phase were calculated, as well as transverse plane angular momentum of all body segments. The shift in pelvis-thorax coordination from in-phase to out of phase with increasing velocity was found to depend on the pelvis beginning to move in-phase with the femur, while the thorax continued to counter rotate with respect to the femur. Moreover, pelvic and thoracic contributions to total body angular momentum were low (less than 10%), while contributions of the legs and arms were much larger (approximately 90%), suggesting that pelvis-thorax coordination is relatively unimportant to the organisation of total body angular momentum. Taken together, these results may imply that our understanding of the pelvic step need to be changed. Moreover, the alterations in pelvis-thorax relative phase that were reported for different locomotor pathologies may depend on different mechanisms.

Gait in Pregnancy-related Pelvic girdle Pain: amplitudes, timing, and coordination of horizontal trunk rotations.

Walking is impaired in Pregnancy-related Pelvic girdle Pain (PPP). Walking velocity is reduced, and in postpartum PPP relative phase between horizontal pelvis and thorax rotations was found to be lower at higher velocities, and rotational amplitudes tended to be larger. While attempting to confirm these findings for PPP during pregnancy, we wanted to identify underlying mechanisms. We compared gait kinematics of 12 healthy pregnant women and 12 pregnant women with PPP, focusing on the amplitudes of transverse segmental rotations, the timing and relative phase of these rotations, and the amplitude of spinal rotations. In PPP during pregnancy walking velocity was lower than in controls, and negatively correlated with fear of movement. While patients' rotational amplitudes were larger, with large inter-individual differences, spinal rotations did not differ between groups. In the patients, peak thorax rotation occurred earlier in the stride cycle at higher velocities, and relative phase was lower. The earlier results on postpartum PPP were confirmed for PPP during pregnancy. Spinal rotations remained unaffected, while at higher velocities the peak of thorax rotations occurred earlier in the stride cycle. The latter change may serve to avoid excessive spine rotations caused by the larger segmental rotations.

The loyal dissident: N.A. Bernstein and the double-edged sword of Stalinism.

Nikolai Aleksandrovich Bernstein (1896-1966) studied movement in order to understand the brain. Contra Pavlov, he saw movements (thus, the brain) as coordinated. For Bernstein, the cortex was a stochastic device; the more cortexes an animal species has, the more variable its actions will be. Actions are planned with a stochastic "model of the future," and relevance is established through blind mathematical search. In the 1950 neoPavlovian affair, he came under strong attack and had to stop experimenting. It is argued that the consistency of his work derived both from both dialectical materialism and the relentless attacks of the neoPavlovians.

How do persons with chronic low back pain speed up and slow down? Trunk-pelvis coordination and lumbar erector spinae activity during gait.

In healthy walking, the timing between trunk and pelvic rotations, as well as erector spinae (ES) activity varies systematically with walking velocity, whereas a comparable velocity-dependent adaptation of trunk-pelvis coordination is often reduced or absent in persons with low back pain (LBP). Based on the hypothesis that trunk-pelvis coordination is linked to overall gait stability, persons with LBP can be expected to have difficulties in dealing with perturbations. We examined the ability of 12 persons with LBP and 12 controls to adapt trunk and pelvis rotations and ES activity to sudden changes in velocity. 3D angular movements of thoracic, lumbar, and pelvic segments and surface EMG were recorded during treadmill walking at six different velocities, which increased or decreased unexpectedly. Relative phases of segmental rotations were determined and (in-)variant properties of kinematics and ES activity were studied using principal component analysis. Compared to healthy controls, persons with LBP exhibited a reduced ability to adapt trunk-pelvis coordination and ES muscle activity to changes in velocity. Altered coordination and muscular control may reflect an attempt to stabilise the spine and prevent the occurrence of unexpected perturbations. The assessment of gait patterns in terms of coordination may help clinicians to quantify movement impairments and may suggest interventions aimed at facilitating the emergence of desired coordination patterns.

Mechanical Perturbations of the Walking Surface Reveal Unaltered Axial Trunk Stiffness in Chronic Low Back Pain Patients.

INTRODUCTION: Patients with chronic low back pain (CLBP) often demonstrate altered timing of thorax rotations in the transverse plane during gait. Increased axial trunk stiffness has been claimed to cause this movement pattern. OBJECTIVES: The objective of this study was to assess whether axial trunk stiffness is increased in gait in CLBP patients. METHODS: 15 CLBP patients and 15 healthy controls walked on a treadmill that imposed rotational perturbations in the transverse plane. The effect of these perturbations on transverse pelvis, thorax and trunk (thorax relative to pelvis) rotations was evaluated in terms of residual rotations, i.e., the deviation of these movements from the unperturbed patterns. In view of the heterogeneity of the CLBP group, we additionally performed a subgroup comparison between seven patients and seven controls with maximal between-group contrast for timing of thorax rotations. RESULTS: Rotations of the walking surface had a clear effect on transverse pelvis, thorax and trunk rotations in all groups. No significant between-group differences on residual transverse pelvis, thorax and trunk rotations were observed. CONCLUSION: Axial trunk stiffness in gait does not appear to be increased in CLBP. Altered timing of thorax rotations in CLBP does not seem to be a result of increased axial trunk stiffness.

Phase-dependent changes in local dynamic stability during walking in elderly with and without knee osteoarthritis.

Previously, we reported reduced time-averaged knee local stability, in the unaffected, but not the affected leg of elderly with knee osteoarthritis OA compared to controls. Since stability may show phase-related changes, we reanalyzed the dataset reported previously using time-dependent local stability, λ(t), and also calculated time-averaged local stability, λs, for comparison. We studied treadmill walking at increasing speeds, focusing on sagittal plane knee movements. 16 patients, 12 healthy peers and 15 young subjects were measured. We found a clear maximum in λ(t) (i.e. minimum in stability) at around 60% of the stride cycle (StanceMax λ(t)), a second clear maximum (SwingMax λ(t)) at around 95% followed by a minimum between 70% and 100% (SwingMin λ(t)). StanceMax λ(t) of both legs was significantly higher in the OA than the young control group. Values for healthy elderly fell between those of the other groups, were significantly higher than in young adults, but there was only a trend towards a significant difference with the StanceMax λ(t) of the OA group׳s affected side. Time-averaged and time-dependent stability measures within one leg were uncorrelated, while time-dependent stability measures at the affected side were inversely correlated with λs at the unaffected side. The results indicate that time-dependent local dynamic stability might provide a more detailed insight into the problems of gait stability in OA than conventional averaged local dynamic stability measures and support the notion that the paradoxical decline in unaffected side time-averaged local stability may be caused by a trade-off between affected and unaffected side stability.

Frontal plane kinematics in walking with moderate hip osteoarthritis: Stability and fall risk.

BACKGROUND: Hip abductor weakness and unilateral pain in patients with moderate hip osteoarthritis may induce changes in frontal plane kinematics during walking that could affect stability and fall risk. METHODS: In 12 fall-prone patients with moderate hip osteoarthritis, 12 healthy peers, and 12 young controls, we assessed the number of falls in the preceding year, hip abductor strength, fear of falling, Harris Hip Score, and pain. Subjects walked on a treadmill with increasing speeds, and kinematics were measured opto-electronically. Parameters reflecting gait stability and regressions of frontal plane center of mass movements on foot placement were calculated. We analyzed the effects of, and interactions with group, and regression of all variables on number of falls. FINDINGS: Patients walked with quicker and wider steps, stood shorter on their affected leg, and had larger peak speeds of frontal plane movements of the center of mass, especially toward their unaffected side. Patients' static margins of stability were larger, but the unaffected dynamic margin of stability was similar between groups. Frontal plane position and acceleration of the center of mass predicted subsequent step width. The peak speed of frontal plane movements toward unaffected had 55% common variance with number of falls, and adding the Harris Hip Score into bivariate regression led to 83% "explained" variance. INTERPRETATION: Quickening and widening steps probably increase stability. Shorter affected side stance time to avoid pain, and/or weakened affected side hip abductors, may lead to faster frontal plane trunk movements toward the unaffected side, which could contribute to fall risk.

PCA in studying coordination and variability: a tutorial.

OBJECTIVE: To explain and underscore the use of principal component analysis in clinical biomechanics as an expedient, unbiased means for reducing high-dimensional data sets to a small number of modes or structures, as well as for teasing apart structural (invariant) and variable components in such data sets. DESIGN: The method is explained formally and then applied to both simulated and real (kinematic and electromyographic) data for didactical purposes, thus illustrating possible applications (and pitfalls) in the study of coordinated movement. BACKGROUND: In the sciences at large, principal component analysis is a well-known method to remove redundant information in multidimensional data sets by means of mode reduction. At present, principal component analysis is starting to penetrate the fundamental and clinical study of human movement, which amplifies the need for an accessible explanation of the method and its possibilities and limitations. Besides mode reduction, we discuss principal component analysis in its capacity as a data-driven filter, allowing for a separation of invariant and variant properties of coordination, which, arguably, is essential in studies of motor variability. METHODS: Principal component analysis is applied to kinematic and electromyographic time series obtained during treadmill walking by healthy humans. RESULTS: Common signal structures or modes are identified in the time series that turn out to be readily interpretable. In addition, the identified coherent modes are eliminated from the data, leaving a filtered, residual pattern from which useful information may be gleaned regarding motor variability. CONCLUSIONS: Principal component analysis allows for the detection of modes (information reduction) in both kinematic and electromyographic data sets, as well as for the separation of invariant structure and variance in those data sets. RELEVANCE: Principal component analysis can be successfully applied to movement data, both as feature extractor and as data-driven filter. Its potential for the (clinical) study of human movement sciences (e.g., diagnostics and evaluation of interventions) is evident but still largely untapped.